The present invention relates to a sound absorber comprising an inner component and an outer component surrounding said inner component, said inner component defining an inner wall provided with apertures, and said outer component defining an outer wall extending along said inner wall.
A sound absorber of this type is known from DE 195 04 223 A1.
Sound absorbers are used in the intake system of engines, e.g. in motor vehicles, for reducing noise emission. The German-Offenlegungsschrift 34 31 078 suggests for this purpose a sound absorber consisting essentially of an intake pipe produced from a sound-absorbing, porous material in a certain section thereof, the intake pipe being surrounded by a perforated metal tube in said section. The above-described sound absorber does not satisfy the requirements on the damping ratio, which are higher than they used to be. What is particularly problematic is the damping of noise emitted by engines provided with turbo-charger systems. When such charger systems are in operation, pulsation noise will occur, which is caused by extremely small geometrical irregularities of a compressor impeller of the turbocharger. This pulsation noise occurs proportionally to the rotary frequency of the turbocharger. The frequency band excited in the case of such noise has, due to the large operating speed range, a very wide bandwidth. Hence, sound absorption over a particularly wide bandwidth has to be effected for achieving a general reduction of noise emission.
In DE 195 04 223 A1, it is suggested that the perforated pipe should be used as an intake pipe, said pipe being surrounded by a cylinder with a closed, flat surface. Between the interior perforated intake pipe and the cylinder concentrically surrounding said intake pipe, a broad, uninterrupted annular gap is formed. The apertures provided in the perforated intake pipe are arranged in the area of said annular gap so as to enable said annular gap to communicate with the interior of the intake pipe. The improved sound absorbing properties of this sound absorber originate form the air mass exchange and pressure compensation with the annular gap, which are made possible by the apertures in the intake pipe; the cylinder defining the annular gap prevents pressure losses and causes a further reduction of noise emission.
However, also the damping ratio of this sound absorber is insufficient for achieving a broadband reduction of noise emission, especially in the case of turbo-charged engines.
DE 196 38 304 A1 discloses a sound absorber which is specially conceived for engines with turbochargers. This sound absorber comprises a chamber arranged in the flow channel and provided with a plurality of spaced-apart annular screens which are arranged in parallel. A respective resonance chamber is formed between two neighbouring annular screens, said resonance chamber leading to a reduction of the sound level in a certain frequency range. This sound absorber is disadvantageous insofar as the screens have edges and that the gas flows across these edges. The flow resistance caused by these edges impairs the efficiency of the charger system. In addition, the insertion of the screens into the sound absorber chamber is complicated from the point of view of production technology and, consequently, it entails high costs.
Furthermore, damping systems comprising a plurality of different damping elements, which are configured to be used for different frequency bands and which are arranged in series, are commercially available and used in engines. Such damping systems, however, require an excessively large accommodation space.
It is therefore the object of the present invention to create a sound absorber which is simple to produce and which has good sound-absorbing properties, in particular broadband sound-absorbing properties.